In existing cellular radio systems, the radio network has a strict control on the behavior of the user equipment (UE). Uplink transmission parameters like frequency, timing, and power are regulated via downlink control signaling from the base station, also referred to as an eNodeB in Long Term Evolution, to the UE.
At power-on or after a long standby time, the UE is not synchronized in the uplink. The UE can derive an uplink frequency and power estimate from the downlink (control) signals. However, a timing estimate is difficult to make since the round-trip propagation delay between the eNodeB and the UE is unknown. So even if UE uplink timing is synchronized to the downlink, it may arrive too late at the eNodeB receiver because of the propagation delays. Therefore, before commencing traffic, the UE has to carry out a Random Access (RA) procedure to the network as illustrated in FIG. 1.
Random access (RA) is the process for the UE to request a connection setup for initial access or to re-establish a radio link. In addition to the usage of RA during initial access, RA is also used when the UE has lost the uplink synchronization in an idle or a low-power mode. Also during a handover process, the RA may be used to setup a connection between the UE and the new base station.
When the UE has transmitted the RA, the eNodeB can estimate the uplink timing misalignment of the UE and send a correction message. During the RA, uplink parameters like timing and power are not very accurate. This poses extra challenges to the dimensioning of a RA procedure.
Usually, a Physical Random Access Channel (PRACH) is provided for the UE to request access to the network. An Access Burst (AB) is used which contains a preamble with a specific sequence with good Auto-Correlation (AC) properties. The PRACH can be orthogonal to the Traffic Channels (TCH). Because multiple UEs can request access at the same time, collisions may occur between requesting UEs. A contention resolution scheme has to be implemented to separate the UE transmissions.
To distinguish between different UEs performing RA typically many different preambles exist. A UE performing RA randomly picks a preamble out of a pool and transmits it. The preamble represents a random UE ID (UE Identity) which can be used by the eNodeB when granting the UE access to the network. The eNodeB receiver can resolve RA attempts performed simultaneously with different preambles and send a response message to each UE using the corresponding random UE IDs. In case that multiple UEs simultaneously use the same preamble a collision occurs and most likely the RA attempts are not successful since the eNodeB cannot distinguish between the two users with the same random UE ID.
In Long Term Evolution (LTE) one or multiple preambles can be derived from a Zadoff-Chu or root sequence. Zadoff-Chu sequences are so called Constant Amplitude Zero Auto Correlation (CAZAC) sequences. This implies a constant magnitude and a perfect periodic auto-correlation function, i.e. the correlation has a single peak at time-lag zero and vanishes everywhere else. This property can now be used to derive multiple preambles from a singe root sequence.
One design goal for LTE has been to create as many unique PRACH preambles as possible. There are 838 root sequences available. To increase the number of available sequences each root sequence can be cyclically shifted (in time) to create more unique sequences. The total number of unique sequences becomes the product of the number of root sequences and the number of cyclic shifts.
The number of possible cyclic shifts is determined by the cell size. The cyclic shift has to be larger in time than the longest propagation delay in the cell plus the largest expected delay spread. The longest propagation delay is dictated by the geographical size of the cell. Thus for larger cells the number of available shifts is less than for small cells, which means that for larger cells the amount of unique preambles becomes less.
In each cell 64 preambles out of the total set are provided. For small cells a few root sequences are used together with short cyclic shifts, in very small cells even a single root sequence is sufficient. Since the needed cyclic shifts are short, there are many possible cyclic shifts and the number of root sequences that needs to be used becomes small since the product of cyclic shifts and number of used root sequences should be at least 64. For larger cells more root sequences are used since there are not so many cyclic shifts possible for each root sequence. Exactly which preambles that are in use in a cell is signaled to the UEs on the broadcast channel.
One constraint for assigning root sequences to the cells is that they should be unique, i.e the same root sequences should not be assigned to the cells close by.
Planning the use of root sequences in a cellular network is a non-trivial task. The number of root sequences used by a cell should be minimized since this allows for larger reuse distance, i.e. the distance between cells that use the same root sequences can be increased. However reducing the number of root sequences used by a cell implies reducing the length of the cyclic shift. However the length of the cyclic shift depends on the size of the cell and it cannot be reduced too much.
Given the irregular shape of cells in practical deployments and variable propagation conditions the planning of PRACH preambles is today done manually.
All preambles derived via cyclic shifting from a single root sequence are orthogonal to each other. Preambles derived from different root sequences are not orthogonal to each other but interfere to other root sequences (and preambles derived thereof via cyclic shifting). Increased interference results in a higher false alarm rate which in turn requires a higher threshold if the false alarm rate should be maintained, leading to a worse missed detection performance. False alarm rate implies that the eNodeB believes that it has found a radio access attempt when it was noise plus interference. The standard governs how high this false alarm rate is allowed to be. In case of higher noise plus interference a higher threshold is needed to maintain this false alarm rate. A higher threshold means that the detected peak must be higher, so the likelihood that true peaks are missed increases.
Yet another reason to keep the number of root sequences used in a cell as small as possible is complexity in the eNodeB. In an eNodeB receiver all preambles derived from a single root sequence can be detected with a single correlator. Preambles derived from another root sequence require an additional correlator, i.e. the number of correlator increase linearly with the number of root sequences used in the cell.
Manual configuration is cumbersome, error prone and costly. A manual configuration would typically not go down to each individual cell but use default configuration values for a whole region. Since these default parameters must work for all cells the values are typically chosen conservative and far from being optimized for individual cells resulting in the problems outlined above.